Frequency multiplexing agc amplifier

ABSTRACT

A circuit for AGC amplification of signals in different acoustic frequency bands so that the signals in all frequency bands are normalized to the signal in one particular band by using a single logarithmic detector responsive to signals in the band to which the other bands are normalized.

United States Patent 1151 Newton 14 1 July 1 l, 1972 [54] FREQUENCYMULTIPLEXING AGC 3,147,459 9/l964 McCarter .330/132 x AMPLIFIER3,209,316 9/l965 Sparks .....330/l44 X 2,870,271 l/l959 Cronburg etal.....................330/52 X [72] Inventor: Victor S. Newton, TakomaPark, Md. 73] Assignee: The United States of I a FOREIGN PATENTS 0RAPPLICATIONS represented by the Secretary of the Navy 951,058 3/1964Great Britain 330/ I 34 {22] Filed: June 1970 Primary Examiner-Roy Lake21 APPL 44,31 Assistant ExaminerJames B. Mullins Attorney-R. S.Sciascia, J. A. Cooke and R. .l. Erickson [S2] U.S. Cl ...330/52,330/132, 330/145 57 TR [5 1] Int. Cl. ..l-l03g 5/16 I 5s 1 Field ofSearch .330/132, 144, 14s, 52 A AGC amPllficamn fl' dmmm mum frequencybands so that the signals in all frequency bands are [56] admin Cmdnormalized to the signal in one particular band by using a singlelogarithmic detector responsive to signals in the band to UNITED STATESPATENTS which the other bands are normalized.

3,360,737 12/1967 Harris et al. 32 X 2 Clalrm, 2 Drawing FiguresLOGARITHMtCALLY DETECTED l6 OUTPUT 30 K 32 A c c BAND PASS BAND PASS AMPOUTPUT FDLTER a FlLTER a 5 as 14' I 3a 2 A s c BAND PASS BAND PASSOUTPUT FILTER A FILTER A A LOGARITHMIC ELEMENT i l L 2 l 46 4 42. I 1 11 l VOLTAGE 0w PASS To CURRENT L DETECTOR CONVERTER F'LTER l l 1FREQUENCY MULTIPLEXING AGC AMPLIFIER BACKGROUND OF THE INVENTION Thisinvention relates generally to acoustic frequency multiplexingcircuitry, and more particularly to a circuit providing automatic gaincontrol (AGC) amplification of signals in different acoustic frequencybands.

In many situations it is desirable to AGC amplify acoustic signals indifferent frequency bands in a manner such that the signals arenormalized to the same level. For example, in identifying a movingobject by its generated acoustic spectrum, or signature. the amplitudesof the acoustic signals in several narrow frequency bands within theobjects wide band spectrum may be compared to determine the overallfrequency characteristic of the spectrum. in this situation, thecomparison is meaningful only if the relative amplitude of the narrowband segments are accurately known. When the entire unknown spectrumvaries over a wide dynamic range of amplitudes, it is necessary tonormalize these amplitudes using AGC circuitry. If each of the severalfrequency segments are not normalized to the same level, it isimpossible to accurately determine the relative amplitudes of thesesegments and, errors will be introduced in the determination of thecharacteristics of the unknown spectrum.

Prior art devices achieve the AGC normalization separately for each ofthe several narrow frequency bands and relay on careful matching of thecircuit components to maintain equal AGC levels. However, when thedynamic range of signal amplitudes under consideration is large, forexample 60 db or more, it is extremely difficult to match AGCcharacteristics over the entire range. Thus, in prior art AGC circuits,using for example, either log diodes or solions as input log detectorsthe voltage-vs.-current characteristics must be matched over the entiredynamic range. This is an extremely difficult and expensive goal toachieve using these circuit devices, requiring great care in theirmanufacture, and testing.

SUMMARY OF THE INVENTION Accordingly, one object of the presentinvention is the provision of an inexpensive and easily produciblecircuit for AGC amplifying acoustic signals in several differentfrequency bands.

Another object of the present invention is to provide an AGC amplifierfor separate acoustic frequency bands having equal gains in every band.

Still another object of the present invention is the provision of asingle AGC amplifier circuit for different acoustic frequency bandsrequiring a single logarithmic element to eliminate matching problems.

Briefly, in accordance with one embodiment of this invention, these andother objects are attained by providing a single AGC amplifier circuitemploying a wide band filter to admit signals from all of the frequencybands of interest, a log detector circuit for converting the widedynamic range input signals into the logarithms of these signals, and anAGC loop which normalizes the input signals in accordance with the levelof signals in one of the frequency bands of interest.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a block diagrammatic view ofone embodiment of an AGC amplifier in which two acoustic frequency bandsare linearly added and then logarithmically detected and AGC amplified;and

FIG. 2 is a block diagrammatic view of an alternative embodiment of anAGC circuit in which all of the narrow frequency bands of interest arepassed through a single wide band input filter, logarithmically detectedand AGC amplified.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings,wherein like reference characters represent identical parts throughoutthe several views, and more particularly to FIG. 1, wherein oneembodiment of the present invention for frequency multiplexing and AGCamplifying acoustic signals in two narrow frequency bands is shown ashaving a transducer 12, such as a microphone or hydrophone, forreceiving a wideband input acoustic signal and converting it into awideband electrical signal. This electrical signal is then applied totwo parallel coupled input band pass filters l4 and 16, one covering anarrow lower frequency band, designated as hand A, and the othercovering a narrow upper frequency band, designated as band B. The twonarrow band signals emerging from filters l4 and 16 are then fed to asumming device 18, such as a resistive network, wherein they are addedand the resultant signal applied to a logarithmic feedback detector 19consisting of a controllable attenuator 20 and an AGC loop 21. Thesignal is first applied to the input of controllable attenuator 20consisting of a resistor 22 and an element 24 having a logarithmicvoltage-vs.- current characteristic, such as a semiconductor diode or atransistor. These two circuit elements act as a voltage divider, whereinthe a.c. voltage at their common junction point 25 is proportional tothe small-signal a.c. resistance of the log element 24, since resistor22 is much larger than the small-signal a.c. resistance of log diode 24.The a.c. signal at junction point 25 is applied to an input capacitor 26in an AGC loop 2]. Capacitor 26 provides leakage current isolation tothe remainder of the AGC loop by preventing any do coupling thereto. Theoutput from capacitor 26 is then applied to a wide band a.c. amplifier28, and thereafter is simultaneously applied to parallel coupled bandpass filters 30 and 36 respectively passing signals in upper frequencyband B, and in lower frequency band A. The output signals from filter 30are then fed to a wide band amplifier 32, which yields AGC outputsignals in upper band B at an output terminal 34. Similarly, the signalsin lower band A emerging from filter 36 are processed through a Widebandamplifier 38 and provide AGC output signals in lower band A at an outputterminal 40.

To efi'ect the AGC function by which the output signals appearing atterminals 34 and 40 are normalized to a constant level, the signals inlower band A emerging from amplifier 38 are fed to a feedback loop 41 inAGC loop 21. A detector 42 therein, such as a true RMS detector oraveraging detector, which detects the a.c. signal received fromamplifier 38. The resulting detected signal is fed to a low pass filter44 which removes any remaining high frequency a.c. components from thedetected signal. This signal is then fed to a voItage-to-currentconverter 46 the output current of which is proportional to its inputvoltage. The output current from converter 46 is used in a negativefeedback manner to current bias the log diode 24 to the appropriatelevel and thereby vary the attenuation of the controllable attenuator 20for maintaining a constant AGC output level. Biasing is done with acurrent rather than a voltage so that the quasi dc. voltage across logelement 24, which is proportional to the logarithm of the a.c. inputsignal in frequency band A at the input to controllable attenuator 20,may be sensed. This logarithmically detected output voltage appears atterminal 27. Because output current from converter 46 is either d.c. orvery low frequency a.c. it is prevented from being fed back through theAGC loop by capacitor 26 and band pass filters 30 and 36.

Thus, in this embodiment it will be noted that AGC amplification ofsignals in two distinct narrow frequency bands is achieved using asingle log detector circuit wherein the amplitudes of signals in bothfrequency bands are normalized to the amplitude of signals in one of thefrequency bands.

FIG. 2 shows an alternate embodiment of a frequency multiplexingamplifier circuit requiring fewer elements than the circuit of FIG. I.In this embodiment, after the acoustic input signal is converted into anelectrical signal by acoustic transducer 12, it is fed to a wide bandfilter 50 which passes signals in both of the narrow frequency bands Aand B. The signal is then processed through controllable attenuator 20and AGC loop 21 as hereinbefore described with respect to the embodimentof FIG. 1.

From the foregoing description of FIG. 2, it will be seen that the AGCcircuit of FIG. 2 requires only a single input wideband filter which isdirectly connected to controllable attenuator 20, while the circuit ofFIG. 1 requires different filters for the separate frequency bands ofinterest and a device for summing the output signals from these filtersbefore they are applied to the controllable attenuators 20. Thus, thecircuit of FIG. 2 performs the same function as the circuit of FIG. 1but is simpler and less expensive.

It will be apparent that the circuits of the instant invention achieveAGC amplification of signals in different frequency bands using the samelogarithmic element for each band, thereby eliminating the need for logelements with matched characteristics. it will also be apparent thatalthough the in vention ha been described in connection with audiofrequency signals and only two narrow frequency bands within a givenrange of frequencies, the invention is equally applicable to otherfrequency ranges and to more than two narrow frequency bands.

Additionally, it will be apparent that a weighted filter may be used forthe signals in the control frequency band A rather than a simple bandpass filter having flat characteristics. By using a weighted filter, itwould be possible to compensate the AGC characteristics in situationswhere the input spectrum in narrow band A was not completely flat.

Obviously, numerous other modifications and variations of the presentinvention are possible in light of the above teachings. [t is thereforeto be understood that within the scope of the appended claims theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. Apparatus for identifying the signature of an object generatingwideband acoustic frequency electrical signals, including a frequencymultiplexing automatic gain control amplifier circuit, comprising:

input band pass filtering means responsive to said wideband acousticfrequency electrical input signal for providing a bandpass electricalsignal from said wideband acoustic frequency electrical signal;

voltage divider means for controllably attenuating said bandpass signalconsisting of a logarithmic responsive element and a resistor;

means for amplifying said controllably attenuated signal, including afirst plurality of filters for separating a first plurality of narrowband signals in said controllably attenuated signal;

feedback means responsive to one of said first plurality of narrow bandsignals for varying the attenuation of said controllably attenuatingmeans in a manner that the amplitudes of all of said first plurality ofnarrow hand signals are normalized to a level determined by theamplitude of said one of said first plurality of narrow band signals;and

output terminals coupled to each of the outputs of said plurality offilters for determining the signature of said acoustic generating objectat said output tenninals;

wherein said one of said narrow band signals is a lower frequencysignal;

wherein said feedback means comprises: means for detecting said one ofsaid first plurality of narrow hand signals and for developing adetected voltage signal; means for removing any remaining high frequencysignals from said detected voltage signal; and means for converting saiddetected voltage signal into a proportionate current signal, wherebysaid current signal is applied to said logarithmic responsive elementfor varying the gain of said controllably attenuating means;

wherein said amplifying means includes a capacitor between saidattenuating means and said first plurality of filters to provide leakagecurrent isolation between said amplifying means and said controllablyattenuating means;ar td wherein said input bandpass filtering meansincludes a second plurality of narrow band filters providing a secondplurality of narrow hand signals, and means for summing said pluralityof narrow band signals.

2. Apparatus as recited in claim 1 wherein said first plurality offilters comprises two bandpass filters, one high pass and one low pass;and

means coupled to said logarithmic element for detecting the output ofsaid logarithmic responsive element.

# 'l' i I!

1. Apparatus for identifying the signature of an object generatingwideband acoustic frequency electrical signals, including a frequencymultiplexing automatic gain control amplifier circuit, comprising: inputband pass filtering means responsive to said wideband acoustic frequencyelectrical input signal for providing a bandpass electrical signal fromsaid wideband acoustic frequency electrical signal; voltage dividermeans for controllably attenuating said bandpass signal consisting of alogarithmic responsive element and a resistor; means for amplifying saidcontrollably attenuated signal, including a first plurality of filtersfor separating a first plurality of narrow band signals in saidcontrollably attenuated signal; feedback means responsive to one of saidfirst plurality of narrow band signals for varying the attenuation ofsaid controllably attenuating means in a manner that the amplitudes ofall of said first plurality of narrow band signals are normalized to alevel determined by the amplitude of said one of said first plurality ofnarrow band signals; and output terminals coupled to each of the outPutsof said plurality of filters for determining the signature of saidacoustic generating object at said output terminals; wherein said one ofsaid narrow band signals is a lower frequency signal; wherein saidfeedback means comprises: means for detecting said one of said firstplurality of narrow band signals and for developing a detected voltagesignal; means for removing any remaining high frequency signals fromsaid detected voltage signal; and means for converting said detectedvoltage signal into a proportionate current signal, whereby said currentsignal is applied to said logarithmic responsive element for varying thegain of said controllably attenuating means; wherein said amplifyingmeans includes a capacitor between said attenuating means and said firstplurality of filters to provide leakage current isolation between saidamplifying means and said controllably attenuating means; and whereinsaid input bandpass filtering means includes a second plurality ofnarrow band filters providing a second plurality of narrow band signals,and means for summing said plurality of narrow band signals. 2.Apparatus as recited in claim 1 wherein said first plurality of filterscomprises two bandpass filters, one high pass and one low pass; andmeans coupled to said logarithmic element for detecting the output ofsaid logarithmic responsive element.